The present invention is related to an image synthesizing method and an imaging apparatus. More specifically, the present invention is directed to an image synthesizing method and an imaging apparatus, capable of obtaining an image signal whose dynamic range has been widened, while both a white balance and a color balance are properly adjusted.
In the case that, for example, an indoor scene is imaged by such an imaging apparatus as a digital still camera, even if good pictures of photographic objects located in an indoor space can be acquired, the blue sky viewed from a window is skipped from an entire image in a white color, so that this entire image may sometimes become unnatural. This is because a dynamic range of the image is narrow. In order to solve this problem, dynamic ranges of images have been conventionally widened by photographing two sets of images and by synthesizing these two images with each other.
For instance, a first short-exposing-time image (low-sensitivity image) is photographed by actuating a shutter in a high speed, and continuously, a second long-exposing-time image (high-sensitivity image) is photographed by actuating the shutter in a low speed, and then, since these two images are synthesized with each other, a scene outside a window, which has been photographed in the low-sensitivity image, may be overlapped with the high-sensitivity image in which an indoor scene has been photographed under better condition.
When such an image synthesizing operation is carried out, since the red (R) signal of the high-sensitivity image can be hardly saturated, as compared with the green (G) signal and the blue (B) signal, the entire high-sensitivity image mainly owns a cyan color. As a result, since the synthesized image is also influenced by the color signal mainly having the cyan color of the high-sensitivity image, there is a problem that the color balance of the entire synthesized image is no longer maintained.
To solve the color balance problem, in the conventional techniques described in, for example, Japanese Patent Publication No. Hei-7-131796, the color signal of one image to be synthesized is multiplied by “k”, and then this k-multiplied image is synthesized with the other image so as to adjust the saturation of the synthesized image.
In the above-explained art, when the color signals of the two photograph images are synthesized with each other, the gray color balance is controlled. However, this conventional technique does not consider shifts contained in white balances of these two images. As a result, the white balance of the high-sensitivity image is different from the white balance of the low-sensitivity image, which are contained in the synthesized image. Thus, there is such a problem that a synthesized image having a sense of incongruity may be formed, depending upon a photographic scene.
Recently, such an imaging apparatus as a digital still camera has been proposed. For instance, while both high-sensitivity pixels and low-sensitivity pixels are mounted on a solid-state imaging element of this digital still camera, a high-sensitivity image (will also be referred to as “high-power image” hereinafter) imaged by the high-sensitivity pixels is synthesized with a low-sensitivity image (will also be referred to as “low-power image” hereinafter) imaged by the low-sensitivity pixels so as to output the synthesized high-sensitive/low-sensitive images as a single image data. When this proposed imaging apparatus performs the image synthesizing operation, executions of not only a color balance, but also a white balance are required.